This invention relates to turning tools and methods and, more particularly, to tools and methods for turning piston skirts to form sculptured bearing surfaces.
It is known in the art relating to machining of piston skirts to form sculptured bearing surfaces in various ways. Among them are grinding of the skirt surface with a dressed grinding wheel shaped to obtain the desired surface configuration. A common configuration appears in cross section as a sine wave and may be formed with spacing between peaks of about 0.50 to 1.50 mm and a depth of 0.005 to 0.05 mm. Another method is to turn the piston skirt with a single point turning tool to obtain a helical pattern of shallow grooves and peaks. A currently used tool has a tip radius of about 0.76 mm and turns the skirt with a helical lead of about 0.29 mm to form shallow arcuate valleys about 0.014 mm deep spaced between angular peaks. With this configuration, the peaks provide a minimum of bearing surface and have been found to wear down, during use of the piston in an engine, to form bearing surface plateaus between the valleys. The plateaus form bearing surfaces that engage a surrounding cylinder and are lubricated by wedge oil films present in the adjacent valleys of the skirt surface configuration. However, the wear may increase the piston-to-bore diametral clearance enough to adversely affect piston secondary motion and noise generation.
The present invention preferably provides a single point turning tool having a tip including a cutting edge with a tip radius the same as or similar to the currently used tool. However, a flat or linear portion is added on one side of and slightly behind the point of the tip radius to form a cutting edge with a linear portion and a protruding portion. The protruding portion consists of that portion of the tip radius that extends beyond the plane of the linear portion which, in turn, extends normal to a longitudinal axis on which the tip radius is centered. In a broader context, the protruding portion could include curvatures other than arcuate and/or could be formed with non curved edges within the scope of the invention. Preferably, the cutting edge also includes lead-in radii or curved surfaces connecting the protruding portion on one side and the linear portion on the other side with angled sides of the tip.
During manufacture of a piston, the turning tool is used to finish the outer surface of the piston skirt. The tool is preferably positioned with the linear portion aligned generally with the cross sectional direction or plane of the finished outer surface. The piston is turned while the tool cuts a helical path or turn around the skirt surface by advancing the tool each turn a desired dimension or lead that is greater than the width of the protruding portion but not greater than the combined widths of the linear and protruding portions. The tool is preferably controlled by a CNC machine which allows the skirt profile to have any desirable shape, such as cylindrical, tapered or barrel shaped with a round, oval or modified transverse cross section. The turning tool may be advanced in either direction along the piston skirt. As the piston is turned, the protruding portion cuts a valley of equal width in the surface and the linear portion cuts an adjacent plateau on the remainder of each turn.
The resulting skirt surface is a series of sequentially adjacent turns each of which has a width equal to the lead of the helical cut. The surface of each turn includes a shallow valley preferably having slightly sloping sides and a width equal to the width of the protruding portion of the tool. Each turn also includes an adjacent flat or linear portion (as seen in longitudinal cross section) having a width equal to the difference between the lead of the helix and the width of the adjacent valley.
In an exemplary embodiment, each plateau has a width of about 34 percent of the total width of the turn (or the lead of the cut). Thus the width of each plateau forms a bearing surface which is about half the width of the adjacent valleys. The valleys are shallow arcs that have slightly sloping sides which, in operation of the piston, form oil film wedges fed to the bearing surfaces in both directions of motion of the piston. A skirt profile having well lubricated bearing surfaces capable of sustaining thrust loads on the piston without excessive break in wear is thus provided by use of the novel turning tool combined with the method of its use.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.